This invention relates to the field of filtration membranes, generally, and specifically to the field of mesoporous membranes, particularly ultrafiltration and diafiltration membranes. The invention also relates to the novel process of using supported porous carbon membranes for selective adsorption and separation.
Membrane filtration technologies are critical to a variety of industrial process applications including cell harvesting, sterilization of biological solutions, clarification of antibiotics, concentration of protein solutions, and particulate filtration. Ultrafiltration is a particular type of membrane separation process that is used to separate macromolecules such as proteins from solutions containing solvents and low molecular weight solutes under the presence of a pressure gradient. Ultrafiltration membranes typically have a pore size from 1 nm to 100 nm. Diafiltration is similar to ultrafiltration, except that changes are made to the solution during processing; in diafiltration the dilution level is typically manipulated during filtration. Membranes used for either of these purposes have the ability to fractionate macromolecular components based upon their individual molecular masses. The typical membrane molecular weight cut-off range is from about 103 to 107 g/mol.
Ultrafiltration is typically carried out with the solution to be processed (rententate) on one side of the membrane and the purified stream (permeate) exiting the system on the other side. The rejected-stream side is operated under higher pressure than the permeate side creating a pressure gradient that drives the solution through the porous membrane structure. The desired component or components remain behind, blocked or retained by their inability to permeate the membrane.
During operation, membrane throughputxe2x80x94the rate at which solution passes through the membranexe2x80x94typically diminishes as the membrane surface becomes fouled with the retained component. Accordingly, the membrane must be periodically cleaned to remove fouling agents, i.e., aggregated proteins, bacterial contamination, etc. This is most commonly performed by exposing the membrane surface to a chemical reagent and back-flushing the system.
Traditionally, ultrafiltration membranes have been primarily polymeric in nature. See Zydney and Zeman (1996), Microfiltration and Ultrafiltrationxe2x80x94Principles and Applications, Marcel Dekker, New York, N.Y. Asymmetric ultrafiltration membranes are commonly synthesized using phase inversion, where a polymer solution of a base and poreformer in a solvent is induced to form two interdispersed liquid phases. After coagulation, a solid membrane gel is produced. Membranes synthesized in this manner include the bilayer type which contains slit-shaped fissures or cracks, see Michaels, U.S. Pat. No. 3,615,024 (1971), and those membranes that contain plasticizers and are stable while dry, see Tweddle and Thayer, U.S. Pat. No. 4,451,424 (1984). See also Cabasso and Levy, U.S. Pat. No. 4,954,381 (1990).
Despite the widespread use of these types of polymeric ultrafiltration membranes, they have several well-known disadvantages. First, the low mechanical stability of polymeric ultrafiltration membranes constrains their maximum operating pressure. The low mechanical stability of polymeric ultrafiltration membranes limits their operating capacity, i.e., maximum permeate flux, because permeate flow is proportional to operating pressure under most conditions. Additionally, the low mechanical stability of polymeric ultrafiltration membranes leads to deformation during operation that can adversely affect membrane performance. Second, polymeric ultrafiltration membranes are particularly sensitive to the harsh reagents and solvents used to remove fouling components. After repeated cleaning, polymeric ultrafiltration membranes typically show signs of degradation. Third, most polymeric ultrafiltration membranes must contain either a humectant, such as glycerol or water, or must be maintained in a saturated state at all times which requires that they be transported and stored in a solvent. Membranes that are unstable with respect to drying or leaching of a humectant are not robust and special considerations, which can be expensive, must be taken during their processing and handling. See Degen et al., U.S. Pat. No. 5,480,554 (1996). Last, mass-produced polymeric ultrafiltration membranes are known to possess cracks and other defects that span the separating layer and limit the performance of these membranes. Curiously, the porous structure of some polymeric ultrafiltration membranes is derived solely from cracking during processing. See Michaels, U.S. Pat. No. 3,615,024 and Degen et al., U.S. Pat. No. 5,480,554.
The supported porous carbon ultrafiltration membranes of the present invention offer many advantages over existing ultrafiltration membranes. The present invention relates to a supported porous carbon membrane having pores in the ultrafiltration range. The carbon membrane is synthesized both within and on top of the macroporous support. The support provides the membrane with high mechanical strength and resists deformation even at high driving force pressures. Deformation due to organic solvent influx, i.e., polymeric swelling, is avoided because the membrane is not polymeric and is strengthened by the rigidity of the support. Because the membrane can operate at higher pressures compared to polymeric membranes, filtration processes using membranes of the present invention can be operated at higher throughput rates.
The carbon membranes of the present invention naturally resist chemical attack during cleaning. In addition to the chemical-based cleaning methods known in the art, the membranes can also be cleaned using either steam sterilization or high temperature desorption because the membranes are stable at high temperatures. Notably, the membranes are stable at temperatures above the melting point of polymeric ultrafiltration membranes.
Carbon membranes are also stable when exposed to air and moisture. The carbon membranes do not require the addition of plasticizing agents or to be handled under a solvent which is necessary for many polymeric ultrafiltration membranes. See Foley (1995), Carbogenic Molecular-Sievesxe2x80x94Synthesis, Properties and Applications, Microporous Materials, 4, 6, pp. 407-433.
The present invention relates to a supported mesoporous carbon membrane where the mesoporous carbon exists both within and external to a structural support, such as porous stainless steel. Currently, the only known examples of supported carbon membranes are used in gas phase separations. These gas-phase membranes have pore sizes in the range of from 0.3 to 1 nm (nanoporous range). The nanoporous carbon membranes are synthesized by the pyrolysis of certain organic and natural polymers. Upon unimolecular reaction at high temperatures, the carbonizing polymers decompose, leaving a nanoporous graphite-like carbon solid. See Foley (1995) at 407-433. The porosity of the polymer precursors is not preserved in the final product. Rather, the porosity results from the carbon membrane""s metastable, graphite-like structure having atomic-size pores. See Acharya and Strano (1999), Simulation of Nanoporous Carbons: A Chemically Constrained Structure, Phil. Mag. B, 79, 10, pp. 1499-1518. Acharya and coworkers have used stainless steel supports to prepare nanoporous gas separation membranes from poly(furfuryl alcohol) resin. See Acharya and Raich (1997), Metal-Supported Carbogenic Molecular Sieve Membranes: Synthesis and Applications, Industrial and Engineering Chemistry Research, 36, 8, pp. 2924-2930; Acharya and Foley (1999), Spray-Coating of Nanoporous Carbon Membranes for Air Separation, Journal of Membrane Science, 161, pp. 1-5. These particular membranes have a remarkable ability to affect small molecule separations such as oxygen and nitrogen extraction from air. See Shiflett and Foley (1999), Ultrasonic Deposition of High Selectivity Nanoporous Carbon Membranes, Science, 285, 17, pp. 1902-1905. However, these types of membranes cannot be used for ultrafiltration purposes because the pores are too narrow (generally less than 1 nm). The small scale of these pores requires transport of liquid feeds across the membrane to proceed by vaporization and adsorption at one boundary of the membrane, then migration of the adsorbed phase across the interior of the membrane, then evaporation at the other surface. Because of this adsorbed phase transport, the nanoporous membranes cannot transport and preserve liquid-phase components across the membrane. Thus, the present invention also relates to the novel application of a supported mesoporous carbon membranes to macromolecular separation.
For efficient ultrafiltration of macromolecular species, it is necessary to control the mode of the pore size distribution of the carbon membrane to range between 1 to 10 nm. All nanoporous adsorbentsxe2x80x94especially those based on carbonxe2x80x94have pores sizes in the range of 0.3 to 1.0 nm and cannot be used for this purpose. It has also never been experimentally verified that carbonizing a polymer-based ultrafiltration membrane can produce a carbon membrane having a pore structure in the ultrafiltration range. In fact, this synthesis route is highly unlikely to be successful because polymeric ultrafiltration membranes typically cannot exist without a plasticizer or humectant, both of which are intrinsically unstable at carbonization temperatures. Additionally, polymeric ultrafiltration membranes require external protection against drying of any kind which cannot be maintained during carbonization.
The present invention further relates to a membrane fabrication process that directs the pyrolysis of a noncarbonizing templating polymer precursor to form an additional pore-size distribution in the mesoporous or ultrafiltration range. The mesoporous carbon membrane is synthesized on a macroporous stainless steel support that provides the resulting membrane with superior mechanical strength compared to existing ultrafiltration membranes. The membranes synthesized according to the present invention were characterized using generally accepted, phenomenology-based techniques. The utility of the mesoporous supported carbon membranes of the present invention was demonstrated with model macromolecular separations.
The process of preparing the novel membranes involves coating a porous metal membrane support with a polymeric precursor composition comprising both a carbonizing polymer and a noncarbonizing templating polymer. The noncarbonizing templating polymer directs the formation of pores in the ultrafiltration range. The coated support is then pyrolyzed in an inert-gas atmosphere. Lafyatis and coworkers found that the addition of certain noncarbonizing polymers such as poly(ethylene glycol) to carbonizing nanoporous-carbon precursors has a pronounced effect on the meso- and macropore structure of carbonaceous adsorbents synthesized through polymer pyrolysis. See Lafyatis and Tung (1991), Poly(Furfuryl Alcohol)-Derived Carbon Molecular-Sievesxe2x80x94Dependence of Adsorptive Properties On Carbonization Temperature, Time, and Poly(Ethylene Glycol) Additives, Industrial and Engineering Chemistry Research, 30, 5, pp. 865-873. In addition to having a pore size distribution mode in the nanopore region (below 1 nm), these materials have a second mode centered in the meso- (1 to 100 nm) to macropore (100 to 1000 nm) regions. Experimentation suggests that the location of this second mode depends directly upon the average molecular weight of the noncarbonizing templating polymer used, for example poly(ethylene glycol), as well as the synthesis conditions. Therefore, the inventors have unexpectedly found that by manipulating the characteristics of these additives, one can synthesize a porous carbon membrane with carefully controlled pore sizes in the ultrafiltration range.
An object of the present invention is to provide a supported mesoporous carbon membrane. Another object of the present invention is the method of producing a supported mesoporous carbon membrane. A further object of the present invention is using a supported mesoporous carbon membrane for the selective adsorption or separation of proteins or other macromolecules in solution.
In accordance with these objectives, the supported mesoporous carbon membranes of the present invention have a controlled pore-size distribution in the mesoporous range making them capable of retaining macromolecules in aqueous solutions. The mesoporosity of the membranes according to the present invention is produced in the carbon layer by the addition of a noncarbonizing templating polymer, such as poly(ethylene glycol) (xe2x80x9cPEGxe2x80x9d), to a nanoporous carbon precursor, i.e., a carbonizing polymer precursor such as poly(furfuryl alcohol) (xe2x80x9cPFAxe2x80x9d), and pyrolyzing the polymeric precursors on a porous support. The pyrolysis of the polymeric precursor mixture on the support produces a templated nanoporous carbon membrane.
The supported mesoporous carbon membranes of the present invention have a composite structure. The membranes of the present invention comprise a mesoporous carbon layer that is located within and/or on top of a porous-metal macroporous support. These membranes have the mechanical strength properties of the porous-metal support and the macromolecular sieving properties of the porous-carbon material. These characteristics are not available in either material separately.
The supported mesoporous carbon membranes of the present invention utilize a porous stainless steel support. In a preferred embodiment, the stainless steel macroporous support has a pore size of from about 0.1 to 100 xcexcm. In a most-preferred embodiment, the stainless steel macroporous support has a pore size of about 0.2 xcexcm.
In a preferred embodiment of the present invention, the supported mesoporous carbon membranes are prepared from a polymeric precursor mixture comprising poly(ethylene glycol) as the noncarbonizing template polymer precursor and poly(furfuryl alcohol) as the carbonizing polymer precursor. In a preferred embodiment, the weight ratio of PEG to PFA is from 1:3 to 3:1. In a more preferred embodiment, the weight ratio of PEG to PFA is 1:1. In yet another embodiment, the molecular weight of the PEG is from 1000 to 18500 amu. In a more preferred embodiment, the molecular weight of the PEG is from 2000 to 8000 amu. In a still more preferred embodiment of the invention, the molecular weight of the PEG is from 3400 to 8000. In a most preferred embodiment, the molecular weight of the PEG is about 8000 amu.
The supported mesoporous carbon membranes of the instant invention have the capacity to permeate liquid solvents under pressure and to sieve, i.e., to retain, particles in the range from 1 to 100 nm. Thus, it is an object of the present invention that the supported mesoporous carbon membranes have an effective or operating pore size of from about 1 to 100 nm. The effective pore size of the mesoporous carbon material is preferably from 1 to 50 nm. The effective pore size of the mesoporous carbon material is most preferable from 1 to 10 nm.
There are numerous industrial applications for the membranes produced according to the present invention. Because the membranes contain a porous-metal support, the membrane support can be prefabricated to very small tolerances and does not require the use of custom-made fittings. Thus, the membrane can be easily and economically incorporated into industrial processes. The porous metal support also permits the membrane to be easily sealed to isolate the two sides of the membrane using commonly-known gaskets or similar devices.
In the field of membrane separation, the attachment of the membrane to the process tubing is crucial to successful operation of the membrane. Because the membrane of the present invention incorporates a porous-metal support, the attachment of the membrane does not require special expertise beyond that which one trained in the art of connecting pieces of metal would be required to have. In a preferred embodiment of the present invention, the membrane is welded to the process tubing. Another preferred embodiment is to connect the membrane to the process tubing using standard compression or vacuum fittings.
In an additional preferred embodiment of the present invention, the support is tube-shaped allowing the membrane to be incorporated into a shell to form a tube-in-shell device. The tube-in-shell device has two zones for fluid flow that are separated by the membrane. To prepare a tube-in-shell device, the membrane is attached, e.g., by welding, at both ends to two lengths of non-porous tubes. To complete the tube-in-shell module, the shell is easily attached to the inner tube using standard compression or vacuum fittings. The membrane module can be used individually, or with several identical units, to provide low energy molecular sieving separations for industry. The module can be assembled and disassembled very quickly, facilitating inspection and replacement of the membrane in industrial applications.
It is a further object of the present invention to provide a separation process using the novel supported mesoporous carbon membrane to separate aqueous protein solutions.
The novel supported mesoporous carbon ultrafiltration membrane of the present invention is further characterized by the following criteria. First, the membrane has a very high mechanical integrity and can be operated at pressures in excess of 1000 psig. Second, the bursting pressure, as rigorously defined, is infinite, i.e., the membrane cannot rupture or deform in the classical sense because the integrity of the membrane is derived from the porous stainless steel support. Third, the membrane is resistant to both mechanical deformation due to higher pressure driving forces and chemical swelling due to organic vapor exposure. Fourth, the membranes are essentially defect-free; the membranes demonstrate total retention of model macromolecular componentsxe2x80x94there is no partial retention observed during the target separation. Fifth, the membranes can be stored either wet or dry and maintain their separation performance in either condition. Sixth, the carbon surface of the membranes is resistant to chemical attack such as that which occurs during cleaning of conventional filtration membranes. Seventh, the membrane is thermally stable at high temperatures, e.g., greater than 200xc2x0 C. enabling the use of high-temperature cleaning and separation processes not possible with polymeric membranes. Notably, the separation properties of the carbon layer have been shown to be temperature independent at temperatures lower than the synthesis temperature. See Mariwala and Foley (1994), Evolution of Ultramicroporous Adsorptive Structure in Poly(Furfuryl Alcohol)-Derived Carbogenic Molecular-Sieves, Industrial and Engineering Chemistry Research, 33, 3, pp. 607-615. Eighth, the membranes are intrinsically sterile after synthesis. Ninth, the membrane contains a nanopore-size pore distribution, as well as an ultrafiltration-size pore distribution. The nanopores function as an adsorbent layer to remove, albeit not continuously, any smaller impurity that can be readily adsorbed on the nanoporous carbon. Last, the membrane can be used in novel separation processes.